Bone is a dynamic tissue which is constantly being remodelled by osteoblasts and osteoclasts. New bone is built by osteoblasts and old bone resorbed by osteoclasts, and the development and homeostasis of skeletal systems depends on this balance between bone formation and resorption.
Insufficient osteoclast activity leads to insufficient amounts of old bone being resorbed and can cause osteopetrosis, a disease in which the bones of the sufferer become denser and harden. Similarly, increased osteoclast activity leads to increased amounts of old bone being resorbed and can also cause disease. Diseases which are associated with increased osteoclast activity include primary and secondary bone cancer, as well as osteoporosis and rheumatoid arthritis.
Osteoclasts express the osteoclast-associated receptor (OSCAR) on their cell surface (WO0220718). This receptor can modulate osteoclast activity (WO0220718) and is also expressed on osteoclast precursor cells (Kim et al., 2002) as well as on the surface of monocytes, macrophages, dendritic cells and neutrophils in humans (Merck, E. et al, 2004; Merck, E. et al, 2005; Merck, E. et al, 2006)).